Tailoring of resins, such as polyethylene homopolymer or copolymer resins is a known method of altering the molecular architecture and thus the bulk properties of the resin and articles made therefrom. Tailoring involves treating the polyethylene resin with an agent, such as a peroxide or oxygen, capable of controlled modification of the resin. The effect of tailoring on the polyethylene resin heretofore has been thought to exclusively influence Theological properties and may generally show an increase in the zero shear viscosity, an increase in elasticity (G′/G″) and an increase in shear thinning behavior of the tailored pellets in comparison to the untailored (granular) product. These changes benefit the process of converting polyethylene pellets into final useful articles by reducing swell during the blow molding process, reducing sag during pipe extrusion, increasing the bubble stability, reducing gauge variation during film conversion.
In WO 03/047839, oxygen tailoring is suggested to increase shear thinning behavior, increase elasticity, increase melt tension, reduce swelling during blow molding, and increase bubble stability during film blowing. This document suggests that this is accomplished using 8 to 40% by volume oxygen in the melt conveying section of the extruder. There is no suggestion to an improvement in optical properties.
In U.S. Pat. No. 5,739,266, modifying a polyethylene in an extruder by bringing the polyethylene in contact oxygen or a gas mixture containing oxygen is suggested. This document suggests that the polymer is contacted with oxygen before it melts, and further suggests that the oxygen contact is performed before complete melting of the polymer. Additionally, this document suggests that the polymer-oxygen contact may occur in any part of the extruder, with the exception of the pumping or melt-conveying zone. The gas mixture suggested in this document contains from 1 to 50% by volume oxygen, when measured in the gas atmosphere of a feed hopper of the extruder. Again, there is no suggestion to an improvement in optical properties.
It would be commercially advantageous to have improved methods of tailoring polyethylene, particularly polyethylene film resin, to provide polyethylene resins having improved optical properties.